Since the discovery of thienamycin in 1976, research on the synthesis of carbapenem antibiotics has been vigorously carried out. Many carbapenem compounds which have excellent antibacterial activity, such as imipenum, have been discovered. However, many of these carbapenem compounds have the drawback that they are easily metabolized by renal dehydropeptidase-I (DHP-I)
Thus, research for improving stability against DHP-I has been vigorously carried and in 1984, the research group at Merck and Company Incorporated developed a 1β-methyl carbapenem compound that, while maintaining excellent antibacterial activity, was also chemically and physically stable as well as having excellent resistance against DHP-I.
Many research groups have since developed 2-(substituted mercapto)-1β-methyl-carbapenem antibiotics in which a substituted mercapto is introduced at the 2-position of the carbapenem skeleton and some of these are in practical use.
As such compounds, L-627 (biapenem) represented by formula shown below:

S4661 (doripenem) represented by formula shown below:

and SM7338 (meropenem) represented by formula shown below:
have been developed, for example.
Conventionally, these 2-(substituted mercapto)-1β-methyl-carbapenem antibiotics are produced by the generally known method represented by reaction scheme shown below:

In formula, R1 represents a pharmaceutically-acceptable ester group or a carboxyl group protective group that can be easily removed; R2 and R3 each represents, independently, a phenyl group that may be substituted with a halogen atom, an alkyl group, a cyano group, a nitro group, or the like, a C1 to C6 alkyl group that may be substituted with a halogen atom, or the like; and X represents a halogen atom such as chlorine and bromine.
In other words, after deriving the phosphoric acid ester compound represented by formula (I) by reacting the compound represented by formula (II) with the compound represented by formula (III) in a suitable solvent such as acetonitrile in the presence of a base such as diisopropylethylamine, the final product is obtained by further reacting with mercaptans.

In above formulas, R1 represents a pharmaceutically-acceptable ester group or a carboxyl group protective group that can be easily removed; R2 and R3 each represents, independently, a phenyl group that may be substituted with a halogen atom, an alkyl group, a cyano group, a nitro group, or the like, a C1 to C6 alkyl group that may be substituted with a halogen atom, or the like; and X represents a halogen atom such as chlorine and bromine.
In the above production method, the compound represented by formula (I) is an important production intermediate in the production of 2-(substituted mercapto)-1β-methyl-carbapenem antibiotics. It is preferable that this compound, particularly in situations where industrial-scale production is assumed, has high purity, is easily handable, and is in a crystalline form.

With respect to the compound represented by formula (I) shown above, Patent Document 1 discloses that the compound represented by formula (Ib) (hereinafter referred to as a “compound (Ib)”) can be isolated in a crystalline form, for example. Also, this document discloses that the compound has excellent stability in the crystalline form and that it is useful as a bulk raw material. However, there are the problems that, by the reacted substrate, the reactivity of the diphenyl phosphoric acid part of the compound (Ib) is relatively poor and the ability to remove phosphoric acid is low.
Also, a p-nitrobenzyl group is used as the carboxyl group protective group in the compound (Ib). This group can be generally easily removed by a normal catalytic hydrogen reduction method using palladium carbon as the catalyst. However, the danger of fire from the filtration of palladium carbon and the use of hydrogen in an industrial process is high and thus the use of a p-nitrobenzyl group is unpreferable. Furthermore, production costs are relatively high when a p-nitrobenzyl group is used. A cheaper, more efficient, and stably-removable protective group is thus desired as the carboxyl group protective group instead of a p-nitrobenzyl group.
Related to the present invention, although Patent Document 2 discloses an example where R1 is an alkyl group in formula (I) shown above, a specific synthesis example is hardly disclosed therein.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 4-330085    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. Hei 4-217985